Filtration can be used in the pharmaceutical, microelectronics, chemical and food industries to provide product and process purity. In these applications, porous membranes can remove particulate, ionic, and other contaminants from fluids. These porous membranes, whose pore size can range from the ultrafiltration (approximately 0.001 μm) to microfiltration (approximately 10 μm), can be made from a chemically compatible and mechanically stable polymeric matrix and have measurable retention, pore size or pore size distribution, and thickness. The size of pores in microporous membranes can range on the order of from about 0.01 to about 5.0 microns, and can be chosen depending upon the particle size or type of impurity to be removed, pressure drop requirements, and viscosity requirements of the application. In use, the porous membranes are generally incorporated into a device which is adapted to be inserted within a fluid stream to effect removal of particles, microorganisms or a solute from process fluids.
Fluid filtration or purification is usually carried out by passing the process fluid through the membrane filter under a differential pressure across the membrane which creates a zone of higher pressure on the upstream side of the membrane than on the downstream side. Liquids being filtered experience a pressure drop across the porous membrane and the membrane is subject to a mechanical stress. This pressure differential can also result in the precipitation of dissolved gases from the liquid; the liquid on the upstream side of the porous membrane has a higher concentration of dissolved gases than the liquid on the downstream side of the porous membrane. This occurs because gases, such as air, have greater solubility in liquids at higher pressures than in liquids at lower pressures. As the liquid passes from the upstream side of the porous membrane to the downstream side, dissolved gases can come out of solution and form bubbles in the liquid and or on porous membrane surfaces. This precipitation of gas is commonly referred to as outgassing of the liquid. Outgassing of a liquid can also occur spontaneously without a pressure differential as long as the liquid contains dissolved gases and there is a driving force for the gases to come out of solution, such as nucleating sites, a change in temperature, or a change in chemical composition that results in the formation of bubbles or gas pockets on the surfaces of a porous membrane. Outgassing liquids typically used in the manufacture of pharmaceuticals, semiconductor devices, and displays can include very high purity water, ozonated water, peroxide containing liquids, organic solvents such as alcohol, and others liquids chemically active, such as concentrated aqueous acids or bases which can contain an oxidizer.
Membrane filtration of these chemically active liquids benefits from the use of a chemically inert filter to prevent membrane degradation and loss of integrity which can result in extractable material being released from the filter during use. Membrane filters made from halogenated polyolefins, for example fluorine containing polymers like polytetrafluoroethylene are commonly utilized in these applications. Fluorine-containing polymers are well known for their chemical inertness and excellent resistance to chemical attack. Fluorine containing polymer membranes have low surface energy, they are hydrophobic, and therefore membranes made from such polymers are difficult to wet with aqueous liquids or other liquids, which have significantly greater surface tension than the surface energy of the membrane. During the filtration of outgassing liquids with a hydrophobic porous membrane, the porous membrane can provide nucleating sites for dissolved gases to come out of solution under the driving force of the pressure differential during the filtration process. Gases which come out of solution at these nucleating sites on the hydrophobic membrane surfaces, including the interior pore surfaces and the exterior or geometric surfaces, can form gas pockets which adhere to the membrane. As these gas pockets grow in size due to continued outgassing, they may begin to displace liquid from the pores of the membrane which can reduce the effective filtration area of the membrane. This phenomenon is usually referred to as dewetting of the porous membrane since the liquid-wetted, or liquid-filled portions of the porous membrane are gradually converted into gas-filled portions.
Dewetting of a porous membrane can also occur spontaneously when a wet membrane, such as a hydrophobic membrane wet with an aqueous fluid, is exposed to a gas such as air. It has been found that this dewetting phenomenon occurs more frequently and is more pronounced in fluorocarbon-based membranes. It has also been found that the rate at which dewetting occurs is greater in small pore size membranes such as 0.2 microns or less, than in larger pore size membranes.
Thus, as the membrane filter dewets with time, it becomes more difficult to purify or filter the same volume of process liquid per unit time as when the filter was newly installed and therefore completely wet. Installation of new filter, re-wetting the dewet filter, or changes in process to compensate for the reduced liquid flow translate into higher operating costs for the user. Re-wetting is time consuming, often utilizes flammable liquids which must be disposed of, and requires flushing which takes time.
PTFE membrane surfaces have been treated to modify their properties and make them more hydrophilic. One method described in International Patent Publication Number WO 03/070784 discloses a method to improve the biocompatibility of a membrane by exposing it to a reactant solution, for example sodium hydroborate and anthroquinone in a solvent like dichloromethane, and exposing the membrane and solution to heat or UV. International Patent Publication Number WO/04/007060 discloses a method to modify PTFE membrane via UV radiation in the presence of Na2SO3 or other chemicals. These methods involve use of sodium containing reagents which can result in metal extractables from the membrane and require extensive flushing of the membrane before use in high purity applications. Another method to make hydrophilic PTFE membranes is to coat PTFE with hydrophilic chemicals as described in U.S. Patent Application Publication No.: 20020144944 and International Patent Publication Number WO/01/58577. Coating porous membranes can be used to change the surface energy of the composite membranes, however the coating is typically applied to a membrane from a solvent, for example an organic solvent, and requires a curing step. This coating process can modify the pore size of the porous membrane and creates chemical waste that needs to be removed and disposed of adding to the cost and time for membrane manufacturing.
Dry methods can be used to modify porous PTFE membranes. U.S. Pat. No. 5,282,965 discloses radio frequency (RF) vacuum plasma treating PTFE membranes at pressures between about 0.01 torr and 10 torr to effect membrane modification and prevent membrane fibril breakage. It was also disclosed that the membrane is placed at an electrode-to-membrane distance of from 1 to 20 cm, to prevent or avoid damage to the membrane surface. It would not have been obvious from this disclosure that atmospheric pressure or porous membrane contact with the electrode could be used to modify membrane without causing fibril breakage or membrane damage that could lead to weakening of the porous membrane.
U.S. Pat. No. 6,074,534 discloses a device for implementing a method of increasing the wettability of a porous body in a microwave generated plasma in vacuum conditions. The porous bodies are made from sintered powders, for example polyethylene, to form marker tips having a pore in the range from 1 micron to about to 50 microns. The disclosure does not disclose compositions or indicate how thin porous membranes, or microporous membrane of relatively inert polymers like PTFE could be treated at atmospheric pressure to form porous materials with stable wettability and high strength.
Using microwave coupled atmospheric pressure non-equilibrium plasma (Shenton et al. J. Polymer Science: Part A, vol 40, 95-109, (2002), and Shenton et al. J. Phys D: Applied Physics, vol 34, 2761-2768, (2001)) Shenton observed surface cleaning (removal of weak boundary layer such as grease, release agents, or surface contamination), crosslinking and branching, and surface-chemical structure modification of 0.1 to 1 mm thick non-porous commodity polymer substrates such as HDPE, LDPE, polypropylene, and PET. Shenton observed enhancement of the surface properties such as the surface energy of the substrates. Shenton et al. did not modify thin porous membranes and did not prepare porous membrane contact wettable materials with long term wettability that were non-dewetting and that substantially retained the base membrane strength.
U.S. Pat. No. 6,709,718 discloses RF atmospheric plasma treatment of porous membranes that include a cavitating agent. The RF atmospheric plasma treatment was reported to improve the hydrophilic properties of the membrane (porous sheet 0.1 micron to 10 micron pores of polypolyolefin including the cavitating agent). Film samples tested for ink infiltration had values less than about 60% indicating that the porous membranes were not completely contact wettable with the ink.
U.S. Pat. No. 5,895,558 discloses atmospheric pressure RF plasma (13.5 MHz) treatment of plastic films, webs, or porous substrates to make them hydrophilic. Short treatment times, for example 15, 8, and 5 seconds were used for spun bonded polypropylene sample to make them water wettable. These materials were not immediately wettable, and appear to require from about 3 to about 10 second absorption times. Short exposure times and use of thick meltblown materials were used to reduce damage to the substrate being treated. It would not have been obvious to use longer treatment times to form hydrophilic materials, especially on thin porous membranes, without causing damage to the membranes.
U.S. Pat. No. 6,118,218 discloses incorporating a porous metallic layer in one of the electrodes of a plasma treatment system. A plasma gas is injected into the electrode at substantially atmospheric pressure and allowed to diffuse through the porous layer, thereby forming a uniform glow-discharge plasma. Organic films such as polypropylene, polyethylene, and polyethylene teraphthalate substrates, commonly used in the food-packaging industry, were treated in the plasma at atmospheric conditions. Various AC-voltage frequencies were used in the 60 Hz to 20 kHz range without noticeable difference in the results. It was disclosed that a steady glow discharge could be produced at substantially lower frequencies than previously possible. Many tests were run routinely with success at 1 kHz, and good glow discharge was produced at frequencies as low as 60 Hz. The surface energy of the modified films appeared to decrease with time following atmospheric plasma treatment, some decreased by up to 20% or more.
T. Kasemura, S. Ozawa, K. Hattori, “Surface Modification of Fluorinated Polymers by Microwave Plasmas,” J. Adhesion, 33: 33 (1990) observed improved wettability of microwave plasma treated film samples with decreasing pressure.